Various types of differential devices are known which function to distribute driving power from a power-transmitting member to left and right driven shafts (see, for example, Japanese Patent Application Laid-Open Publication No. HEI-8-318745 (hereinafter referred to as a “related patent document”).
FIG. 15 hereof is a sectional view of the conventional differential device for an agricultural working machine disclosed in the related patent document. In the disclosed differential device, a rotatable primary power-transmitting member 81 has an axial hole 81c in which are inserted respective one end portions of two, i.e. left and right, axles 4 (functioning as output shafts A) rotatable relative to each other about a rotational centerline (rotation axis) of the primary power-transmitting member 81. Thus, driving power is distributed from the primary power-transmitting member 81 to the left and right axles 4 via left and right engaging/disengaging mechanisms 8. The left and right engaging/disengaging mechanisms 8 include left and right secondary power-transmitting members 82 that are mounted on the left and right axles 4, respectively, in opposed relation to opposite axial end surfaces (circumferential recessed end surfaces) 81b of the primary power-transmitting member 81. The primary power-transmitting member 81 also has three horizontally-elongated through-holes 81a formed at predetermined circumferential intervals, in each of which is accommodated two axially-opposed power-transmitting balls 80, resiliently biased away from each other in the axial direction by means of a coil spring 80a with a regulating pin 80b interposed therebetween. Each of the secondary power-transmitting members 82 has ball-engaging grooves 82a each provided for holding one of the balls 80. Reference numeral 29T represents a sprocket integrally formed on the primary power-transmitting member 81, 82b a washer, and 82c a retaining ring.
When a load acting one either one of the axles 4 has exceeded a predetermined value, the corresponding balls 80 associated with the axle 4 are displaced from their engaged positions in the respective engaging grooves 82a to their released positions, to thereby automatically cut off the power transmission from the primary power-transmitting member 81 to the axle 4. With such a differential device, the working machine can be appropriately turned to any desired direction by a human operator only operating the machine so that a traveling resistance to the inner wheel, located at an inner side of a turning track, (i.e., the left wheel when the vehicle is turning left, or the right wheel when the vehicle is turning right) becomes greater than a predetermined value; as a consequence, it is possible to eliminate a need for the human operator to manipulate any particular lever.
However, because the engaging/disengaging mechanism 8, employed in the agricultural working machine disclosed in the related patent document, is arranged to automatically cut off the power transmission to the axle 4 once the traveling resistance to the corresponding wheel has exceeded the predetermined value, there is a possibility of the engaging/disengaging mechanism 8 being automatically activated at a time unintended by the human operator, and thus a further improvement has to be made to ensure satisfactory traveling performance of the working machine.